The present invention relates to a solid state imaging device used for reading of picture elements in a facsimile machine etc., and more particularly relates to improvement in linearity of photoelectric conversion characteristics.
The typical imaging device includes a phototransistor receptive of incident light effective to induce electric charges which are stored in a base region of the phototransistor. The stored charges are read out externally. The conventional solid state imaging device is constructed as follows. Namely, the phototransistor has a base region regularly held in floating state. Its emitter electrode is connected to a resetting MOS transistor, and also connected to a switching MOS transistor for feeding outside a photo-electrically converted signal. These three transistor elements constitute together one bit unit. A plurality of the bit unit are arranged linearly to form a linear image sensor. The linear array of the bit units is sequentially scanned by a controlling circuit provided in the image sensor.
Each of the phototransistors is sequentially returned to an initial state by means of the resetting transistor through the emitter of each phototransistor. Thereafter, electric charges are generated by irradiation of incident light, and are stored in the base region of each phototransistor. After passing a predetermined time interval corresponding to one scanning period, each corresponding switching MOS transistor is sequentially turned on to feed externally from an output terminal through a signal line an electric signal which is obtained by the photoelectric conversion in each phototransistor. In such construction, a light receiving element is composed of the phototransistor which can generate a relatively great output signal due to its amplification function to thereby advantageously increase sensitivity, as compared to another construction where the light receiving element is composed of a photodiode.
However, the phototransistor has normally an optimum operative range to perform linear amplification function when the phototransistor is placed in forward bias condition in which more than a predetermined potential difference is applied between the base and the emitter of the phototransistor. In turn, when the potential difference therebetween is less than the predetermined level, the phototransistor cannot perform sufficient amplification function. Therefore, the conventional image sensor has nonlinear photo-electric conversion characteristics, as a whole, which exhibits the nonlinear relation between the light exposure amount and the output signal voltage magnitude. Namely, the conventional image sensor has relatively low sensitivity characteristics in relatively small light exposure amount range, thereby causing the drawback that a constant sensitivity cannot be ensured throughout the entire range of the light exposure amount.